The present invention relates to outdoor or high bay illumination. In particular, the present invention relates to reflectors used with high intensity arc lamps.
Lamps send out light in all directions. Without a shade or reflector, this light is only blocked by the base of the lamp. In order to make better use of this light, the lamps are used with reflectors and lenses. The reflectors reflect light emanating from one side of the lamp to another direction, and lenses refract the light so that it travels in a different and preferred direction when it passes from a lens than the direction it traveled when it entered the lens.
Light is a form of energy that should be conserved and not wasted. Light can also become a form of pollution if it is directed where it interferes with other activities, such as when light intended to illuminate the ground is partially directed into the sky only to obscure the stars. Control of a light beam is therefore important for three reasons, (1) light should be directed where it is needed so that it does what it is intended to do: illuminate an area, for example; (2) it should not be wasted by directing it where it serves no purpose; and (3) it should not be directed where it interferes with other activities.
Controlling light from lamps is not as simple as it might sound. Especially when using lamps to illuminate large areas of ground outside at night. The goal of outdoor illumination, generally speaking, is to spread light from a simple lamp over as wide an area as possible and as evenly as possible but not to allow any light to be directed upwardly or across grounds where it is not needed or desired.
For example, see the lighting optical system of Jones as described in U.S. Pat. No. 4,591,960 which is intended to uniformly illuminate a large area using multiple reflectors. FIG. 3 of Jones shows a candle power distribution curve achieved by his system. This type of curve is commonly used to compare the efficiencies of lighting systems.
Often light is directed upwardly at an object in such a way that little of the light actually reaches the object and much is directed into the sky where it serves no purpose or blocks the view of the stars. An example of this is illumination of flags. Meyer in U.S. Pat. No. 3,752,975 teaches a light at the top of a flag pole for illuminating the flag from above rather than below but his light does not provide the lateral range for illuminating an unfurled flag.
Parabolic reflectors are well known in illuminating systems, especially those where the light is to be directed downwardly. Parabolic reflectors by definition tend to keep light rays parallel when the lamp located at the focus of the parabola so that a parabolic reflector does not disperse light but tends to illuminate a small area. If the parabola is directed downwards and its edge is extended below the focus of the parabola and the lamp, it will prevent light from being directed upwardly. See for example, the device described by Thompson in U.S. Pat. No. 5,329,438.
Many lights use visors or reflectors that are less focused than that shown in Thompson""s device to allow some spread of the light in a horizontal direction.
Nonetheless, there: remains a need for a reflector that better achieves the goal of an even, downward-only, widespread light pattern, especially for use outdoors and in high bays.
According to its major aspects and broadly stated, the present invention is a reflector having a parabolic cap in combination with a high intensity discharge lamp, in particular one with a vertically-oriented arc tube, placed so that the lowest point of the arc tube is approximately even with the bottom edge of the cap. In an alternative embodiment, a portion of the cap may extend below the lowest point of the arc tube. Under either embodiment, the glass bulb that encloses the arc tube will extend below at least a portion of the edge of the cap so that, when the present lamp reflector is viewed from the side, the bottom of the glass lamp will be visible but the arc tube in the glass lamp will be just eclipsed by the edge of the cap.
By forming a reflector having this shape and placing a lamp at the designated location, the distribution of light from the lamp is extraordinarily even and wide and light is not directed above the plane defined by the cap""s edge.
The parabolic shape described herein is substantially parabolic; that is, one where the first portion of the cap, as measured from its center, may initially be flat but its shape is otherwise a true parabola through most of its length. The bottom of the arc tube should be no lower that the bottom of the cap.
A feature of the present invention is the combination of the position of the lamp with respect to the reflector and the reflector""s shape. Because of this physical relationship, no light will be directed upwardly and much of the light will be directed laterally. The present reflector and lamp illuminate as wide an area as current prismatic lensed lamps with high efficiency but send no light upward; prismatic lamps send as much as 30%-35% of their light upward. Furthermore, there are no lenses to clean with the present invention.
Another feature of the invention is the use of a parabolic cap and an arc tube placed below the focus when the cap is in its normal orientation, namely, with a vertical axis and is downwardly-directed opening. By making the cap parabolic and locating the arc tube as described herein, light striking the inside of the cap toward its edge is reflected laterally to a greater extent than if the cap were a different shape.
Other features and advantages of the present invention will be apparent to those skilled in the art from a careful reading of the Detailed Description of a Preferred Embodiment presented below and accompanied by the drawings.